1. Field of the Invention
The present invention relates to a laser device which wavelength-converts laser beams emitted from either a semiconductor laser having a plurality of luminous points, or a semiconductor having a longitudinal multimode, a laser display apparatus, a laser radiating apparatus, and a nonlinear optical element.
2. Description of the Related Art
A laser light source is useful as a light source for a projection display which has a wide color gamut, high color purity and a high efficiency. Thus, the laser light source can provide an image which is of high quality in terms of a color reproducibility, minute expression, a local luminance and the like. However, since a laser beam emitted from the laser light source is a highly coherent light, when the laser beam is radiated to a surface to be radiated, a speckle noise due to coherency is generated, thereby causing deterioration of the image quality.
With regard to a method of suppressing the speckle noise, heretofore, there have been proposed a method of driving a laser at a high frequency, and a method of vibrating either a diffusion plate or a diffusion sheet. In addition thereto, a method using an external optical element has been proposed. The method is typified by a method of causing a laser beam to pass through each of fibers having different lengths once, thereby reducing coherency. Moreover, there has been proposed a method using a semiconductor laser array for emitting a low-coherence beam in which an oscillation wavelength width is spread up to about 4 nm.
Of these various methods, the method of spreading the wavelength by using the semiconductor laser array has an advantage that the speckle noise can be reduced with an inexpensive device structure because it is not accompanied by an increase in number of members and scale-up of a driving circuit.
Now, with regard to the lasers, a solid-state laser, a liquid laser, a gas laser, a semiconductor laser, a free electron laser, and the like are known depending on media thereof. First of all, the semiconductor laser has the properties, such as a compact size, lightweight, and low power consumption, which are suitable for the laser light source for the display. However, a semiconductor material adapted to emit a laser beam which has a green wavelength band of the three primary colors, red, green and blue, and which is practical enough in terms of the efficiency and the reliability is not yet developed at present.
For this reason, there is mainly used a technique for waveform-converting a laser beam having a wavelength in an infrared wavelength band by using a nonlinear optical element, thereby obtaining a laser beam having a wavelength in a wavelength band of green. For example, a laser beam having a waveform in a 1,060 nm band and emitted from either a semiconductor laser or a Diode Pumped Solid State Laser (DPSSL) is used as a fundamental wave. Also, a green laser beam having a wavelength in a 530 nm band can be obtained by forming a second-order harmonic in a nonlinear optical crystal such as lithium niobate (LiNbO3) or lithium tantalite (LiTaO3). This technique, for example, is described in JP-T-2008-522445 (hereinafter referred to as Patent Document 1).
In addition, a wavelength converting device is proposed in which a periodical modulation structure having a plurality of periods in a traveling direction of a light is provided in a nonlinear optical medium for wavelength-converting excited lights having a plurality of wavelengths and made incident thereto, thereby making it possible to carry out the design corresponding to the arbitrary number of waveforms of the excited lights. This wavelength converting device, for example, is described in Japanese Patent No. 3,971,708 (hereinafter referred to as Patent Document 2).
Moreover, a laser device for wavelength-converting a laser beam having a longitudinal multimode is proposed as a light source for the purpose of reducing the speckle noise described above in the laser device for carrying out the wavelength conversion in such a manner. This laser device, for example, is described in Japanese Patent Laid-Open No. 2007-173769 (hereinafter referred to as Patent Document 3).